This invention relates to information storage devices and, in particular, to an erasable and reusable archival memory utilizing a highly focused laser beam for recording, reading and erasing.
Since the early 1960's, it has been apparent to those involved with the recording art that optical recording, and in particular, optical disc recording, is a very promising method because it allows instantaneous playback, very fast random access, and much higher recording density than is possible with magnetic recording, and archival storage. It is widely recognized that the optical disc system with the greatest potential is the type that uses a highly focused laser beam as an ultra-fine recording stylus to store encoded information at very high data rates with extremely high density.
In pursuit of the vast potential offered by this method, the laser recording art has progressed to the point where there are numerous different recording processes contending to provide the first practical laser recording system. However, in spite of all the activity in the field, no one has come forward with a practical system that is economical enough to be marketed on a large scale. The reason is that each and every recording process known has at least one significant flaw or disadvantage.
The recording process must have a high enough resolution to allow the recording of marks of very small size to satisfy the requirement of high packing density, and the sensitivity of the process must be high enough to permit the use of economical, low power recording lasers. But the sensitivity should be low enough so that the recording medium can withstand a playback exposure high enough to yield a high signal to noise ratio. Furthermore, it is clear that in order to avoid long term degradation due to ambient conditions, and to ensure totally non-destructive readout, the process must have a well defined exposure threshold below which there is no response. In addition, it is desired that the recording process should permit instant playback without the need for post-recording processing before readout. None of the presently known processes meet all these requirements.
One known method involves using heat to induce a stable transition between two structural states having different optical properties, namely the amorphous state and the crystalline state. In the prior art, the only materials known to be useful in this process were the amorphous semiconductors and the chalcogenides.
One type of reversible mass optical memory based on these structural changes involves a photon enhanced transformation from the amorphous state to the crystalline state. The major drawbacks of this process are that it has relatively low sensitivity (typically 10.sup.-2 J/cm.sup.2) low resolution (typically 500 lines per mm maximum) limited by the size of the crystalline grains, and slow write times which are limited by the inherent delay due to the crystallization process. This delay severely limits the recording rate because the transition to the crystalline state depends on the presence of the recording beam.
In a further process using amorphous thin films of chalcogenides, a crystalline track is first made with a continuous wave laser, and the bits are written by amorphizing with the pulsed laser. This is called a reverse mode process. Erasing is done by moving the track under a continuous wave laser spot of low power. A bulk erase is also possible by heating up the entire film above the glass forming temperature of about 120.degree. C. There is a difficulty with the continuous wave laser erase: the degree of crystallization in the track tends to increase after each erase, which causes an increase in the amount of amorphizing energy required. Thus, if a large number of erases are required, a method of stabilizing the track density must be found. Reverse mode writing has the advantage that writing is done by the fast amorphizing process--the speed is limited by the laser not the film. In forward mode, writing is by the slower crystallizing process, which requires about 1 .mu.sec per bit.
These processes employing semiconductor and chalcogenide films rely on changes in the optical transmission and reflectance properties that occur when a transition takes place from one morphological state to another. As these changes are not very great, a high signal to noise ratio is not obtainable with these processes.